Chevy 366 bus carburetor secondaried not opening

HI all. New to the forum but this looks like a good place to ask. I have a 1983 GMC C600 school bus. I use it occasionally to haul. I'd noticed for a while now that when trying to get over 40mph it accelerates slow at this point. Maintaining over 35-40 is hard to do as well. Starting out in lower gears she pulls up fine. Its getting into 4th where she falls on her face. I have great vacuum according to the gauge and it hasn't changed from before. I checked for vacuum leaks and did tune up with plugs and wire. No effect. Then I checked the vacuum secondaries. Its the stock holley 4150EG. When I open the throttle by hand under the hood the secondaries move freely. I pulled the secondaries vacuum linkage from the carb. When I work the diaphragm and plug the vacuum ins and outs the rod stays up and doesn't leak down. I put my manual vacuum pump to it and this lifts the linkage no problem. I re-installed everything and tried the paper clip trick to see if the secondaries would engage but still no luck. I am sure I have no vacuum leaks so I don't know what else to try. I can get a rebuld kit for the carb but other than the secondaries not opening it runs great.The kits are $35. Or I can get a used Edelbrock performer 600cfm which is vacuum secondaries for around $100. I've heard this is a good upgrade fora carb on this engine but not sure which I should do. Rebuild and it still might not work or just slam on a different carb and hopefully be done with it.

Check for any interference between the throttle body and throttle plates and between the air horn and air valve (if so equipped).

I once owned a 1979 Trans AM with TA 6.6 Pontiac 400 CI engine and 4 speed transmission.

Due to an intermittent stumble upon acceleration (another issue entirely and quite a subtle problem in itself), I rebuilt the Rochester Quadrajet carburetor (no help, as it did not need rebuilding). Upon reassembly, I followed the German, goodentight practice when tighteninig the screws attaching the hair horn to the float bowl, overtightening them in the process.

The added stress caused the air horn casting to deform over the next several months, eventually resulting in an interference between the secondary air valve and the air horn. The interference was extremely minor, resulting in no visible evidence to the naked eye, but was enough to prevent the secondary air valve from opening in certain conditions, depending upon temperature, etc.

The air valve seemed to always work properly when trying to find the gremlin but would hang up in the worst possible situations.

It took me over a year to find the gremlin but 10 seconds with a Dremel tool solved it once found.

Wish I still had the two 1979 WS6 TA 6.6 Trans AMs that I once owned, especially the 10 year anniversary model with 1,200 miles.

Dean

Try the Edelbrock. I've had good luck with them. Simple to adjust.

Sounds like the vacuum passage to the secondary diaphragm is plugged or blocked somehow, possibly a wrong gasket. I can't recall how the secondary gets it's vacuum, but by tracing the port to it's source, you should be able to find the blockage.

There may also be a secondary lock out associated with the choke. Some lock out the secondary until the choke is fully open.

There are advantages and disadvantages to the Holly/Edelbrock swap.

I personally detest Holly carbs! Difficult to tune, prone to leak gas. Did I mention they are prone to leak gas?

The advantage to the Holly, assuming it is the factory carb, and you say it runs well other than the secondary problem, it is already tuned and jetted to that engine.

My opinion, the Edelbrock is a better carb, but if you put it on, you will need to modify the throttle linkage, modify the fuel line, somehow make the choke work, and go through the tuning process if you intend to get it to perform properly and get all the gas mileage it can get.

Ditto regarding the secondary lock out.

Most (all?) OEM carburetors with secondary venturis had temperature actuated secondary lock out systems.

Dean

IIRC Edelbrock is just putting there name on a Holly, Carter or quadrajet. Selling all 3 designs. So you could end up with the same style carb.

Bingo.

Edelbrock bought Carter (or the rights) several rears ago. It appears that they bought rights (or companies) to Holley and or Rochester more recently.

FWIW, the Edelbrock carburetors commonly available for several years are Carter AFB, AVS, etc. designs, often install as OEM equipment on semi and high performance engines since the early 1960s.

The AFB and AVS designs were quite sophisticated and easily modified for various engines and states of tune by changing jets, metering rods, metering rod springs, etc.

Holley designs are easier to work on and gained the reputation for HP in the 1960s but are much less sophisticated, less economical and much more prone to leakage.

I tuned many of both in the late 1960s/early 1970s.

Dean

I am a bit out of turn here due to lack of truck engine carb knowledge but if I get your question right you are not sure if the secondary's are opening and thus causing your power problems? On the vacuum secondary Holleys that were used on cars there was a slotted cam and link under the vacuum diaphragm that served to pull the secondarys closed when you let off the throttle. An old hot rodder trick was to put a screw in that slot so the link to the throttle shaft would also jerk the secondarys open mechanically. With no accelerator pump back there it will cause a bit of a stumble whenyou mash the throttle but it will answer your questions as to if they are not opening and causing your problems.

Well without seeing the carb in question and the engine i can not just up and nail down your problem for sure . some truck engines have a vac. gov. some don't and along with that they have sorta a gov built into the dist. SOOOOOooooo you could be having a carb issue with the power valve . there could be and issue with the secondary's it could be a problem in the dist. and making it a timing issue . even just running on the primary side it should run on up with decent power . some people plum hate Holley's and complain on how hard they are to tune , myself i have never had a problem with them . It is what you have worded with and ran as to how well you can make it work for you . Some people can only make a Chevy go , then there are the ones that can make a Ford or MO PAR knock a Chey's socks into the creek . So to make a Holley work ya have to have the correct power valve , the correct jetting the correct fuel level the correct acceleration pump and setting and correct vac secondary spring , then ya go playen from there to shell we say ENHANCE it's performance .

I owned severl of those trucks years ago. They were older than yours, sounds like 1 of the o rings has dried out and letting it suck air. When you get it right, if the rest of the motor is good, it will set you back in the seat.

THe vacuum for the secondary diaphragm comes through passages that are ported from the venturies. I blew those passages out with carb cleaner when I had the diaphragm housing off the carb. Again, the secondaries move freely by hand when I have the throttle open so I don't think its a clearance issue. I am sure the secondaries aren't opening at any point when the engine is running. The carb has an electric governor. THe previous owner had disconnected the wiring from the governor and actually removed it so I wouldn't even know where to wire it up. I read that the only thing the governor would do is actully close the secondaries to limit rpms. I don't think it would have anything to so with actually opening them. Just in case I left the arm of the mechanical part of the governor disconnected from the piece that would control the secondaries so it cannot physically interfere with the secondaries now no matter what. The only part I forgot to check is the there could be a bad seal between the vacuum diphragm housing and the carb itself. To check that I need to get some rubber hose on the tips of a needle nose piers, press the hoses with the pliers over the vacuum passages inside the venturies, then press up on the vacuum diaphragm rod and see if it leaks between the diaphragm housing and the carb.

As to the edelbrock carbs, the performer is actually a Carter AFB. I just happen to look on CL a bit ago and see a guy who has two edelbrock performer 1407's. THose are manual choke and vacuum secondaries and 750 cfm's. Probably work fine on this application. Anyways one is still NIB and the other came off his 54 ford. He said it looks new and was working fine but his son swapped it for a fuel injection kit. I'm snagging the pair for $300. That's less than the price of one new so score for me.I'll still check to see if that little gasket is bad. If I can't get it working on the Holley I'll swap on the Edelbrock. If I don't end up using the Edelbrock that's okay because its a good deal and I'm sure I can get into something that I can use it on soon.

THe vacuum for the secondary diaphragm comes through passages that are ported from the venturies. I blew those passages out with carb cleaner when I had the diaphragm housing off the carb. Again, the secondaries move freely by hand when I have the throttle open so I don't think its a clearance issue. I am sure the secondaries aren't opening at any point when the engine is running. The carb has an electric governor. THe previous owner had disconnected the wiring from the governor and actually removed it so I wouldn't even know where to wire it up. I read that the only thing the governor would do is actully close the secondaries to limit rpms. I don't think it would have anything to so with actually opening them. Just in case I left the arm of the mechanical part of the governor disconnected from the piece that would control the secondaries so it cannot physically interfere with the secondaries now no matter what. The only part I forgot to check is the there could be a bad seal between the vacuum diphragm housing and the carb itself. To check that I need to get some rubber hose on the tips of a needle nose piers, press the hoses with the pliers over the vacuum passages inside the venturies, then press up on the vacuum diaphragm rod and see if it leaks between the diaphragm housing and the carb.

As to the edelbrock carbs, the performer is actually a Carter AFB. I just happen to look on CL a bit ago and see a guy who has two edelbrock performer 1407's. THose are manual choke and vacuum secondaries and 750 cfm's. Probably work fine on this application. Anyways one is still NIB and the other came off his 54 ford. He said it looks new and was working fine but his son swapped it for a fuel injection kit. I'm snagging the pair for $300. That's less than the price of one new so score for me.I'll still check to see if that little gasket is bad. If I can't get it working on the Holley I'll swap on the Edelbrock. If I don't end up using the Edelbrock that's okay because its a good deal and I'm sure I can get into something that I can use it on soon.

Could be wrong here, but "4150EG" sounds like it's an Electronic overspeed Governor model.

The vacuum solenoid part of that is circled in red in the photo.

Even if yours doesn't have one now, it likely originally had one, someone may have unsuccessfully tried to eliminate it by putting a standard cover over the diaphragm, causing the issue you are having.

Perhaps???

<img src = "http://i167.photobucket.com/albums/u125/27Grainfield/Stuff/HolleyEG_zpse0sidxid.jpg">